Conventionally, the outer surface or the inner surface of a cylindrical workpiece made of stainless steel, plain carbon steel, non-ferrous metal, etc. has been mirror-polished according to the turning or grinding method with the use of a super-precision lathe or a grinder in which diamond tools are disposed. According to such method, however, an amount of metal in a workpiece to be removed by the tools has been extremely small and the polishing speed has therefore been low. Furthermore, since mechanical precision such as the deflection of the machine tool main body or thermal deformation of a workpiece has exerted an influence directly upon machining precision, it has been required to provide particular measures for maintaining machining precision to a predetermined degree. In view of the foregoing, such conventional method has not been suitable particularly for mirror-polishing a large-sized cylindrical workpiece.
Therefore, a large-size cylindrical workpiece has conventionally been polished by using a hand polishing tool. However, according to such method, not only a working efficiency has been low, but also it has been difficult to obtain a clean finished surface having high precision and to perform a polishing operation with a low cost.
Electrolytical polishing has also been proposed, but since ordinary electrolytical polishing has required an electrolyte of a strong acid, the working efficiency has been low. Moreover, mechanical prearrangements have been required.
There has also been proposed a method according to which a water-permeable abrasive matter has been disposed at and an electrolyte has been supplied to the gap between working electrodes and the outer surface to be treated of a cylindrical workpiece, and the cylindrical workpiece has been rotated, and a direct current or a pulsating voltage has been supplied to the gap between the working electrodes and the cylindrical workpiece to generate anodic passive films on the surface to be treated, and projected portions of the surface to be treated have been removed by abrasion and electrolytical liquation. According to such method, however, the finishing ability of the abrasive matter has been lowered with the lapse of time, resulting in the insufficiency of the finishing amount of the surfaces to be treated. There has been proposed to replace a used abrasive matter with a new one with the finishing operation interrupted. In such a case, however, an increase in finishing force due to the replacement of the abrasive matter has generated excessive finishing traces on the surface to be treated, whereby a uniformly finished surface could not be obtained. In addition, the working efficiency has been decreased.
According to any of the conventional methods, the minimum surface roughness to be obtained is 0.2 to 0.3 .mu.m Rmax, and it is difficult to obtain a uniform surface having surface roughness of 0.1 .mu.m Rmax or less, and it is not possible to obtain the roundness of a cylindrical workpiece of a few .mu.m or less.